Amplifier Power - Quest Engineering QTA 6060M User Manual

QTA 6000 Series
User Manual
15% discrepancy can mean that a 100 watt
system will only be able to power 7 or
8 x10 watt speakers safely. Be aware of
this booby trap. Always plan to have 20%
more power than you think you will need.
2. Incorrect output connection: If you ac-
cidentally connect your terminal strip
on the amplifier to the 8 ohm output
instead of the 100/70V line, you will have
distortion and risk damaging the amplifier..
3. Short circuits and no circuits: Check that
your wiring has not been accidentally cut,
miss-connected or generally damaged
in the course of installation. This can be
common with building sites with multiple
trades people installing equipment into the
same ceiling cavities as the audio system.
How to calculate the correct
number of speakers and what
wattage connection for a given

amplifier power.

18
If you connect too many speakers to an
amplifier you will have distortion, overheating
of the amplifier and generally poor
performance. The problem is not really "too
many speakers", it is more a problem of "too
much wattage draw exceeding the output
capability of an amplifier. A similar problem
would be trying to draw 2,000 watts from
a 1,000 watt generator. Sooner or later....
Expect a system failure.
Let us take a 100 watt amplifier. If you have
20 speakers and you set them at the 5 watt
taping, you will have a total 100 watt draw
which is the maximum output of the amplifier.
This is correct in theory but in practice you
will be safer connecting 18 speakers, not 20.
The reason is that most ceiling speakers draw
more than their claimed wattage.
The alternative would be to connect the
speakers to the 4 watt transformer terminal
giving you a theoretical total draw of 80 watts
and thus, plenty of "headroom". One watt
difference is not very noticeable as far as
output sound pressure level in concerned.
User Manual Q-Tech Commercial Series
QTA 6060M/6120M/6250M
A system that does not distort will give
much clearer voice reproduction than a
louder system that is distorting or loosing
the sibilant frequencies .
There is a formula for testing the potential
power of the system by measuring the
impedance of each speaker and then adding
all the figures to a total impedance, which is
then compared to the amplifier's expected
impedance/power figures. The quick way is
to just add up all the wattages and then give
yourself 10-20% headroom by reducing the
number of speakers per amplifier or lowering
the wattages slightly.
For the benefit of those who want to calculate
the power of the system with a formula. See
below:
Determining power by calculating
total impedance
The alternative to counting the speaker taping
watts is to calculate the total impedance of
the line, which will also indicate the wattage
necessary to drive the system correctly.
The computational formula is voltage squared
divided by impedance:
Power = Voltage2 / Impedance .
When the speaker line reaches its top voltage
of 70 Volts, the formula is:
Power = 5000 / Impedance
squared equals 5000).
To measure impedance a dedicated meter is
required. These use a 1k frequency to send an
alternating current through the transformer(s).
The resulting figure indicated by the meter's
dial can be cross referenced with the meter's
impedance chart to see what value should be
expected and the power that will be required.
(because 70